Recently, efforts directed toward the growth of diamond at low pressures, where it is metastable, have increased dramatically. Although the ability to produce diamond by low-pressure synthesis techniques has been known, drawbacks including extremely low growth rates prevented wide commercial acceptance. Recent developments have led to higher growth rates, thus spurring recent industrial interest in the field. Additionally, the discovery of an entirely new class of solids, know as "diamond-like" carbons and hydrocarbons, is an outgrowth of such recent work.
Low pressure growth of diamond is an example of what has been dubbed "chemical vapor deposition" or "CVD" in the field. Three predominant CVD techniques have found favor in the literature. One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen wherein the hydrocarbon content usually is varied from about 0.05% to 1.5% in the atomic ratio of carbon to hydrogen. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree. to 2400.degree. C. The gas mixture disassociates at the filament surface and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate (often molybdenum) is heated to a temperature in the region of about 400.degree. to 1100.degree. C. U.S. Pat. No. 4,434,188 which describes in detail a CVD process of causing diamond nucleation and growth from a heated gas mixture in contact with a substrate.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density, growth rate, and it is believed to enhance formation of diamond films as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems. One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
The third method in use is direct deposit from acetylene as a hydrocarbon rich oxyacetylene flame. In this technique, conducted at atmospheric pressure, a specific part of the flame is played on a substrate on which diamonds may condense at rates as high as 100 microns/hr or more. See Y. Matsui, A. Yuuki, M. Sahara, Y. Hirose, Japan Journal of Applied Physics, vol. 28, p. 178 (1989).
In general, processes for the chemical vapor deposition of diamond involve selection of operating parameters such as the selection of a precursor gas and diluent gases, the mixture proportions of the gases, gas temperature and pressure, the substrate temperature and means of gas activation. These parameters are adjusted to provide diamond nucleation and growth on a substrate. Mixture proportions and conditions must provide atomic hydrogen to stabilize the surface of the diamond film and preferably minimize the deposition of graphite. Codeposition of graphite is more evident if the hydrocarbon (methane) concentration is increased above about 3%.
It is well known that CVD diamond tends to nucleate on certain substrate materials more readily than others and that good bonding to the substrate is necessary during the growth period, particularly when growing thick films. The diamond film grows in tension due to growth defects and the "intrinsic stain" induces a force which is proportional to the film thickness. Good bonding is necessary to avoid catastrophic release of the film as a result of this intrinsic strain. However, the diamond film can be so strongly attached to the substrate that at the end of the growth period it can not be removed without dissolving the substrate or where there is a significant differential in thermal expansion between the diamond and the substrate, the diamond film may crack during cool down. The use of release agents will promote the eventual removal of the film from the substrate but may cause the diamond to be so poorly bonded during growth that it causes a catastrophic release. Substrates of molybdenum have been favored in producing thin diamond films because the CVD diamond tends to nucleate readily on this material. However, removal of thick films from molybdenum substrates has posed problems due to strong carbide bonds which cause cracking on cool down and/or require dissolution of the substrate to obtain self-supporting films.
It is desirable to produce thick CVD diamond films which are easily removed from a substrate, do not release prematurely during deposition and do not crack upon cool down.